A printed circuit board (PCB) is a copper-clad substrate, which serves as a foundation for installing electronic components thereon. Generally, a copper-clad substrate includes at least one insulation layer made of a dielectric material, and a conductive layer made of copper foil and arranged on the insulation layer for forming traces (copper conductors). An insulation layer is mostly made of paper, bakelite, fiberglass, rubber, and other types of insulated material impregnated with resin. For convenience of description hereinafter, the insulation layer used in a copper-clad substrate is referred to as a dielectric material layer in the present invention.
With the increasingly complex and diverse circuit design requirements, printed circuit boards have evolved from a single-layer board and a double-layer board to a multi-layer board. Currently, multi-layer printed circuit boards include multiple layers of dielectric material and multiple layers of conductive material, so that more complicated and diversified circuits can be formed. The dielectric material layers may define a plurality of through holes, the walls of which can be plated with conductive material to be connected to traces of each layer, so that the board can be made smaller while accommodating more electronic components. FR-4, FR-5, FR-6 and FR-7, which are common substrates on the market, can be used as a core material for multi-layer PCBs.
As electronic devices continue to be miniaturized, some electronic components for special requirements are moving towards higher power. Under this condition, higher heat is prone to accumulate in smaller space. Also, there is a requirement for the spacing and width of traces to be reduced. For example, for a board made of bakelite or fiberglass, the distance between traces is reduced to about 50 micrometers (μm), and this makes the problem of high temperature worse due to heat accumulation on a circuit, which is difficult to deal with.
High-power components consume a large amount of electrical energy. This means they may have higher work efficiency, but it is inevitable that a certain percentage of energy is converted into heat energy. Furthermore, electronic technology is moving towards the complexity and diversification of electronic circuit design and layout. When high-power electronic components are placed on a printed circuit board, it means that more energy-consuming components will work on an identical or even smaller space. Under this trend, the heat problem will be more difficult to deal with than ever. Since the insulation layer of a copper-clad substrate of a general printed circuit board is mostly made of a dielectric material, which is not a good thermal conductor, the thermal energy generated by a high heating element is accumulated close to the element, and this makes the operation environment of the board unsatisfactory. Furthermore, too much heat accumulation usually leads to the expansion of a printed circuit board. However, the thermal expansion coefficient of a printed circuit board is different from those of the electronic components. As a result, the board can be damaged due to thermal stress.
Currently, in order to improve the heat dissipation efficiency, there are two commonly used methods. One method is thermal convection or radiation, which allows the thermal energy generated by the electronic components of a printed circuit board to spread out to the ambient environment around the board. However, the thermal dissipation efficiency of this method is low. Another method employs a metal wire or heat sink, which has good thermal conductivity. Although this method has a better effect on thermal dissipation than a board which dissipates heat only via its dielectric material, the thermal dissipation efficiency is not high due to the diameter of the metal wire or the dimension of the heat sink being not adequate. On the other hand, a heat sink is usually attached to a printed circuit board via a thermal conductive adhesive, which has a thermal-conductive coefficient far less than a metal. Even a cooling fan is installed at one side of a heat sink remote from the electronic components, the thermal dissipation effect is compromised.
A third method is adding a heat pipe. However, it occupies large space and is complicated in structure. Thus, the manufacturing cost may be increased significantly. Other methods involve modifying the material or structure of a printed circuit board. For example, aluminum has thermal conductivity of about 237 W/m/K and can be used as a metal core of a printed circuit board (known as Metal Core PCB or MCPCB). However, due to technical reasons, this method is not yet to be used for making a multi-layer board. This method is not suitable for a complicated circuit design. Furthermore, this method may cause layout deformation on a board during a manufacturing process, and thus cannot be used widely.
Currently, the commonly adopted solution is to use a ceramic material as an insulation layer of a printed circuit board. The most commonly used ceramic material is aluminum oxide (Al2O3), which can be used on a DBC (direct bonded copper) board, wherein the thermal conductivity of single-crystal aluminum oxide can reach 35 W/m/K; the thermal conductivity of poly-crystal aluminum oxide ranges from 20 to 27 W/m/K. Other commonly used ceramic materials include: aluminum nitride (AlN), beryllium oxide (BeO), and silicon carbide (SiC). Since the above-mentioned ceramic materials with good thermal conductivity are commonly used in circuit boards accommodating high-power electronic components, such boards are sometimes referred to as high-power electronic substrates or boards.
In practice, if a printed circuit board made of a ceramic material is to be used, the width of the traces can be as small as 30 micrometers (μm). However, the ceramic board is usually manufactured under high temperature, a small amount of uneven expansion or bending may occur, so that the ceramic boards are not as good as general printed circuit boards in precision and not suitable to be manufactured as multi-layer boards. On the other hand, metal atoms constituting the circuit are easily diffused during the high temperature process, so that the distance between the traces must be maintained at about 80 micrometers (μm). Therefore, in addition to an increased cost, the distance between the traces cannot be reduced, and the traces cannot be formed at precise location, so that an electronic device employing a whole-piece ceramic board cannot be miniaturized.
Therefore, for a high heat-generating electronic component, the component is often arranged on a small ceramic piece to form a package, and then the package can be provided on a resin-type printed circuit board to form a stacked structure. However, this may increase the volume of the board. Besides, there exists a material of poor thermal conductivity between the ceramic piece and a heat conducting element, the originally asserted thermal dissipation efficiency can be compromised.
Some manufacturer has proposed a semiconductor process to deposit heat-dissipating materials on an etched dielectric substrate to form a composite heat-dissipating substrate, as shown in FIG. 1. However, the equipment for conducting the semiconductor process is expensive, and this raises the cost of a printed circuit board, wherein a mask apparatus accounts for a large proportion of the cost. Therefore, this method is not suitable for products which are diversified and made in small quantity. Since this method is subjected to considerable limitations, it is not accepted by general board makers.
Therefore, the present invention intends to provide a printed circuit board, which allows the width and spacing of the traces of a circuit to be small compared to conventional boards, so that circuit designs can be miniaturized, the thermal dissipation efficiency can be increased to be suitable for an application requiring high-power electronic components and suitable for products which are diversified and made in small quantity to provide flexibility in manufacturing to meet the requirements of different customers.